Kant granted only a limited acceptance to the empiricist view of the natural dimensions of human life, because he thought that in addition to being bound by the laws of nature, human beings are also rational beings whose actions are governed by their sense of reason. He posits that in addition to occupying the natural dimension, the human beings occupy, what he calls, the intelligible realm. In this intelligible realm, reason is the guide for action—here we don’t understand events by their physical causes but by examining the reasons.
If our physical nature were the sole determinant of our actions, then our actions would be reactions. By proposing the idea that the will is autonomous, Kant ensures that moral judgements are applicable to actions. There can be no scientific proof of the autonomy of the will, or our freedom, because freedom is a concept that is reached through our intuitive awareness of the moral law. The concept of freedom is based on reason; it is not a scientific proof. We are morally autonomous by virtue of being rational and not because of our physical condition.
In Kant’s philosophy, there are two kinds of reasons for acting in one way or other: hypothetical imperatives and categorical imperatives. A hypothetical imperative is based on inclination or desire—Kant says that it represents "the practical necessity of a possible action as means to something else that is willed (or at least which one might possibly will).” A categorical imperative is based on reason; not tied to any inclination or desire, it has its own moral authority—Kant says it represents "an action as objectively necessary in itself apart from its relation to a further end.”
The categorical imperative is controversial because several scholars are of the view that it can be misused to commit all kinds of monstrous acts. It has been suggested that a dictator may invoke the categorical imperative to justify the execution of his political opponents. But this view is based on a misunderstanding of the Kantian argument. Any vile end, like that of a dictator ordering the execution of his opponents, is tied to personal inclinations and desires, and hence it cannot be seen as categorical; it should be seen as hypothetical.
There are enough provisions in the Kantian moral theory to rule out vile acts being justified as a categorical imperative or universal law. For instance, Kant posits this version of the categorical imperative, “Man is never merely a means to an end, but always an end unto himself.” This means that Kant’s theory disallows the use of another person as a means or tool for the achievement of any end—to do so would be tantamount to denying that person the moral autonomy on the basis which we pass moral judgements, and it would also mean that you qualify for the same treatment.